The present invention relates to a cross dipole antenna suitable for being installed in telecommunication equipment employing circularly polarized waves, and to a composite antenna suitable for being used in a communication system employing both circularly polarized waves and linearly polarized waves.
Various proposals have been made on satellite communication systems for the purpose of mobile communication employing circularly polarized waves. As the satellite communication system, there are a geosynchronous mobile satellite system employing a geosynchronous satellite and a non-geosynchronous mobile satellite system employing a non-geosynchronous satellite.
As the non-geosynchronous mobile satellite system, there are a system employing a low/medium-earth orbit satellite, a system employing a highly elliptical orbit satellite and a system employing an inclined geosynchronous orbit. Among the above, there is the LEO (Low Earth Orbit) communication system as the system employing a low/medium-earth orbit. This LEO communication system is a system having a small propagation delay time. Moreover, as the propagation loss is also small, there is an advantage in that the transmission power can be reduced and it is easy to miniaturize the size and lighten the weight of the terminal.
In addition, with this LEO communication system, there are a small scale LEO (Little LEO) for handling only data transmission and a large scale LEO (Big LEO) capable of voice transmission. The Iridium system and ICO (Intermediate Circular Orbit) system (Project 21) are included in this Big LEO. The communication method of the Iridium system is a TDMA (Time Division Multiple Access) method employing a frequency in a 1.6GHz band, and conducts communication with (66+6) non-geosynchronous satellites launched to an altitude of 780 km so as to cover the entire globe. These non-geosynchronous satellites are disposed at longitudinal 30xc2x0 intervals for orbiting. In addition, the ICO system disposes 6 orbiting satellites in orthogonally inclined orbits of 10390 km, respectively, and the portable terminal thereof is a dual terminal capable of sharing satellite system networks utilizing satellites and existing ground system mobile phone systems.
With such satellite mobile communication systems, although numerous satellites are required, real-time voice and data communication is possible since the delay time can be disregarded. It is further possible to make the terminal portable since the transmission power of the terminal can be reduced. Thus, carrying a portable wireless device of such satellite mobile communication system will realize real-time communication and data transmission with telephones and mobile phones around the world. Circularly polarized waves suitable for portable wireless devices is employed in satellite mobile communication systems.
Incidentally, across dipole antenna or micro strip antenna capable of transmission and reception is employed in a portable wireless device of such satellite mobile communication systems since it is necessary to receive circularly polarized waves.
A cross dipole antenna is structured from two half-wavelength dipole antennae in which dipole antennae are orthogonally disposed in a cross shape. By mutually shifting the phase of two half-wavelength dipole antennae 90 degrees and exciting the same, circularly polarized waves are generated in a direction perpendicular to the face of the two half-wavelength dipole antennae. Here, as mutually opposing circularly polarized waves are generated in the two directions perpendicular to the face of the two half-wavelength dipole antennae, it is standard to place a reflecting plate at the position of approximately xc2xc wavelength rearward of the two half-wavelength dipole antennae for unidirectional use. Further, in order to obtain circularly polarized waves within the range of a wide elevation angle, employed is an inverted V-shaped or inverted U-shaped dipole antenna which shows small directional change in the electric field face and magnetic field face.
FIG. 20 and FIG. 21 show a fundamental structure of a conventional cross dipole antenna capable of transmitting and receiving this type of circularly polarized waves. FIG. 20 is a diagram showing the fundamental structure of a cross dipole antenna 100 employing an inverted V-shaped dipole antenna, and FIG. 21 is a diagram showing the fundamental structure of a cross dipole antenna 200 employing an inverted V-shaped dipole antenna.
The cross dipole antenna 100 employed in the inverted V-shaped dipole antenna shown in FIG. 20 is structured of a reflecting plate 106, an inverted V-shaped first dipole antenna formed of dipole elements 102a, 102b disposed on such reflecting plate 106, and an inverted V-shaped second dipole antenna formed of dipole elements 102c, 102d disposed approximately orthogonal to the first dipole antenna.
This cross dipole antenna 100, although not shown, comprises a phase shifter circuit in the inverted V-shaped first dipole antenna and inverted V-shaped second dipole antenna for mutually shifting the phase approximately 90 degrees and exciting the same. The cross dipole antenna 100 can thereby be used as a an antenna capable of transmitting and receiving circularly polarized waves, and it can further obtain circularly polarized waves in a range of a wide elevation angle since it is formed of an inverted V-shaped first dipole antenna and an inverted V-shaped second dipole antenna.
The cross dipole antenna 200 employed in the inverted U-shaped dipole antenna shown in FIG. 21 is structured of a reflecting plate 206, an inverted U-shaped first dipole antenna formed of dipole elements 202a, 202b disposed on such reflecting plate 206, and an inverted U-shaped second dipole antenna formed of dipole elements 202c, 202d disposed approximately orthogonal to the first dipole antenna. This cross dipole antenna 200, although not shown, comprises a phase shifter circuit in the inverted U-shaped first dipole antenna and inverted U-shaped second dipole antenna for mutually shifting the phase approximately 90 degrees and exciting the same. The cross dipole antenna 200 can thereby be used as an antenna capable of transmitting and receiving circularly polarized waves, and it can further obtain circularly polarized waves in a range of a wide elevation angle since it is formed of an inverted U-shaped first dipole antenna and an inverted U-shaped second dipole antenna.
Since the aforementioned cross dipole antennae are capable of transmitting and receiving circularly polarized waves, they may be employed in communication systems utilizing circularly polarized waves, such as satellite communication antennae and so on. Next, FIG. 22 and FIG. 23 show the concrete structure of the conventionally proposed cross dipole antenna capable of transmitting and receiving circularly polarized waves. FIG. 22, however, is a plan view of the cross dipole antenna and FIG. 23 is the front view thereof. This cross dipole antenna may be installed in automobiles, ships and vessels, aircraft, portable devices, and so forth.
The cross dipole antenna 300 illustrated in these diagrams is structured of two dipole antennae disposed to be approximately orthogonal and a reflecting plate 306. The diameter D3 of the approximately circular reflecting plate 306 is approximately xcex/2 to xcex when the wavelength of the center frequency in the used frequency band is set to xcex. The two dipole antennae disposed to be approximately orthogonal are structured from a first inverted U-shaped dipole antenna and a second U-shaped dipole antenna being orthogonally disposed. The first inverted U-shaped dipole antenna is structured from a dipole element 302a and a dipole element 302b, and the second inverted U-shaped dipole antenna is structured from a dipole element 302c and a dipole element 302d. Dipole elements 302a to 302d are formed of metal plates, and the approximate center thereof is folded toward the reflecting plate 306, and the end thereof is directed toward the reflecting plate 306. The length L302 of dipole elements 302a to 302d is approximately xcex/4.
In this cross dipole antenna 300, the length L301 between one end of dipole elements 302a to 302d and the reflecting plate 306 is set be approximately xcex/4. In other words, the length from the reflecting plate 306 of a coaxial semi-rigid cable 304a for exciting the first inverted U-shaped dipole antenna structured from dipole element 302a and dipole element 302b is approximately xcex/4. Similarly, the length from the reflecting plate 306 of a coaxial semi-rigid cable 304c for exciting the second inverted U-shaped dipole antenna structured from dipole element 302c and dipole element 302d is also approximately xcex/4. Moreover, the length from a short pole 304b and short pole 304d in which the lower end thereof is short-circuited to the reflecting plate 306 is also approximately xcex/4.
One end of the dipole element 302a is connected to and excited by a covered conductor at the tip of the coaxial semi-rigid cable 304a, and one end of the dipole element 302b is connected to and excited by the tip of the short pole 304b. A center conductor 302e of the coaxial semi-rigid cable 304a is connected to the tip of this short pole 304b. Further, one end of the dipole element 302c is connected to and excited by a covered conductor at the tip of the coaxial semi-rigid cable 304c, and one end of the dipole element 302d is connected to and excited by the tip of the short pole 304d. A center conductor 302f of the coaxial semi-rigid cable 304c is connected to the tip of this short pole 304d. 
Moreover, coaxial semi-rigid cables 304a, 304c penetrating through and extending below the reflecting plate 306 are connected to a phase delay circuit 307, coaxial semi-rigid cable 304a is excited at 0xc2x0 phase, and coaxial semi-rigid cable 304c is excited at a 90xc2x0 delayed phase. Thereby, the phase of the first inverted U-shaped dipole antenna and the second inverted U-shaped dipole antenna differ at approximately 90xc2x0, and circularly polarized waves are irradiated pursuant to the excitation from a feeder unit 308.
FIG. 24 illustrates the directivity characteristic inside the perpendicular face of the cross dipole antenna 300 structured as described above. Upon reviewing this directivity characteristic, it is clear that the antenna gain gradually decreases and the axial ratio of the circularly polarized waves deteriorates and becomes an elliptical polarization in the direction of a low elevation angle in which the angle becomes larger from the apex direction.
As described above, with a conventionally proposed cross dipole antenna, the antenna gain decreases and the axial ratio of the circularly polarized waves also deteriorates in the direction of a low elevation angle. This constitutes a problem in a communication system employing circularly polarized waves.
In other words, there are cases where radio waves arrive from the direction of a low elevation angle in a communication system employing circularly polarized waves. Particularly in a satellite communication system, a satellite is generally not geosynchronous and the apparent movement speed of the satellite in a position where the elevation angle is high becomes large. This implies that the existing possibility of a satellite in a low elevation angle of approximately 70xc2x0 to 90xc2x0 upon setting the zenith direction to 0xc2x0 becomes high. Thus, a conventional cross dipole antenna has a problem in that the transmission gain is small in a low elevation angle where the existing possibility of a satellite is high, and the axial ratio deteriorates as well.
Meanwhile, a satellite digital sound broadcast system for conducting digital sound broadcast utilizing satellites has been proposed. FIG. 25 illustrates the schematic structure of this satellite digital sound broadcast system.
As shown in FIG. 25, the satellite digital sound broadcast system transmits digital sound broadcasting programs produced by a plurality of providers from the earth station 171 to the broadcasting satellite 170, and transmits such programs to the assigned territories on earth from the broadcasting satellite 170 based on the control of the ground-side controlling station. Radio waves of the digital sound broadcast transmitted from this broadcasting satellite 170 are circularly polarized waves, and are received by a movable mobile body 182. Here, in the cities where skyscrapers are standing side by side, blind areas may arise because radio waves from the broadcasting satellite 170 do not reach such areas.
Thus, in order to enable favorable reception of sound broadcasting by the mobile body 182 in the cities where blind areas easily arise, terrestrial broadcasting is conducted from the earth broadcasting station 181. The digital sound broadcasting programs broadcast from the earth broadcast station 181 are the same as the digital sound broadcasting programs broadcast from the broadcasting satellite 170, and the terrestrial broadcasting and satellite broadcasting are broadcast in synchronization. Moreover, terrestrial broadcasting is transmitted in linearly polarized waves from the earth broadcasting station in order to suppress interference. Transmitted to the earth broadcasting station 181 are digital sound broadcasting programs broadcast terrestrially from the ground-side controlling station (not shown) and digital sound broadcast programs from the earth station 171. Further, it is possible to obtain digital sound broadcast programs broadcast terrestrially from a satellite broadcast transmitted from the broadcasting satellite 170. The frequency band of the terrestrial broadcasting is made identical or adjacent to the frequency band of the satellite broadcasting.
The mobile body 182 capable of receiving the satellite broadcast or terrestrial broadcast is equipped with an antenna 182a having a circular polarization antenna and linear polarization antenna, and selects and receives a favorable reception by detecting the reception power and so on of both broadcasts. This type of satellite digital broadcast system has been put into practical application as Sirius satellite radio and XM satellite radio.
A circular polarization antenna capable of receiving circularly polarized waves are required for a mobile reception terminal to receive the digital sound broadcast transmitted from the broadcasting satellite 170, and a linear polarization antenna capable of receiving linearly polarized waves are further required upon receiving digital sound broadcasts in cities where blind areas easily arise. That is, two antennae; namely, a satellite system antenna and a ground system antenna, are required.
The cross dipole antenna illustrated in FIGS. 20 to 23 described above is an antenna capable of receiving circularly polarized waves. Nevertheless, although this type of cross dipole antenna is capable of receiving circularly polarized waves and linearly polarized waves, the transmission gain decreases in comparison to an antenna dedicated to linearly polarized waves with respect to the linearly polarized waves transmitted horizontally from the earth station. Therefore, regarding the antenna in a mobile reception terminal in a satellite digital sound broadcast system illustrated in FIG. 25, there is a problem in that it is necessary to separately install a ground antenna such as a whip antenna, for example, in addition to installing a satellite antenna such as a cross dipole antenna.
Thus, the first cross dipole antenna of the present invention comprises: a reflecting plate; a first dipole antenna disposed at a prescribed interval on the reflecting plate; a second dipole antenna disposed at a prescribed interval on the reflecting plate so as to be approximately orthogonal to the first dipole antenna; and a plurality of non-feeding elements disposed around the first dipole antenna and second dipole antenna and uprising from the reflecting plate.
According to this type of invention, since a plurality of non-feeding elements are provided so as to be disposed around the approximately orthogonal first dipole antenna and second dipole antenna and uprising from the reflecting plate, it is possible to suppress the decrease of gain in a low elevation angle and to significantly improve the axial ratio characteristic of circularly polarized waves. In other words, the non-feeding elements act as the wave director and improve the antenna characteristic in the direction of the low elevation angle.
Moreover, in the aforementioned first cross dipole antenna of the present invention, the first dipole antenna and second dipole antenna may be structured by being folded toward the reflecting plate.
Furthermore, in the aforementioned first cross dipole antenna of the present invention, the non-feeding elements may be fixated on the reflecting plate via insulation spacers.
Next, the second cross dipole antenna of the present invention comprises: a reflecting plate formed in which the reflecting face is inclined such that the center portion protrudes further than the peripheral portion; a first dipole antenna disposed at a prescribed interval on the reflecting plate; and a second dipole antenna disposed at a prescribed interval on the reflecting plate so as to be approximately orthogonal to the first dipole antenna. By forming the reflecting plate such that the peripheral portion is inclined downward so as to be positioned lower than the center portion, it is possible to suppress the decrease of gain in a low elevation angle and to significantly improve the axial ratio characteristic of circularly polarized waves.
Moreover, in the aforementioned second cross dipole antenna of the present invention, the dipole antenna and second dipole antenna may be structured by being folded toward the reflecting plate.
Furthermore, the aforementioned second cross dipole antenna of the present invention may further comprise a plurality of non-feeding elements disposed around the first dipole antenna and second dipole antenna and uprising from the reflecting plate.
In addition, in the aforementioned second cross dipole antenna of the present invention, the non-feeding elements may be fixated on the reflecting plate via insulation spacers.
Next, the composite antenna of the present invention is a composite antenna in which a cross dipole antenna capable of receiving circularly polarized waves and a whip antenna capable of receiving linearly polarized waves of an identical or adjacent frequency band to such circularly polarized waves are provided on a reflecting plate; wherein the cross dipole antenna is formed of a first dipole antenna disposed at a prescribed interval on the reflecting plate and a second dipole antenna disposed at a prescribed interval on the reflecting plate so as to be approximately orthogonal to the first dipole antenna; the whip antenna is fixated on the reflecting plate by being isolated from the cross dipole antenna at more than approximately xc2xc wavelength of the wavelength in the center frequency of the used frequency band; and the cross dipole antenna is capable of receiving broadcast signals of circularly polarized waves transmitted from a satellite and the whip antenna is capable of receiving broadcast signals of linearly polarized waves of identical contents as with the broadcast signals transmitted from the ground.
According to this type of invention, since a whip antenna capable of transmitting and receiving linearly polarized waves is provided on the reflecting plate structuring the cross dipole antenna, installation of a single composite antenna will enable the reception of both linearly polarized waves and circularly polarized waves. Therefore, upon receiving digital sound broadcast with a mobile reception terminal, it is no longer necessary to install two antennae; namely, a satellite system antenna and a ground system antenna, and a single composite antenna will suffice.
Moreover, another composite antenna of the present invention is a composite antenna in which a cross dipole antenna capable of receiving circularly polarized waves and a whip antenna capable of receiving linearly polarized waves of an identical or adjacent frequency band to the circularly polarized waves are provided on a reflecting plate; wherein the cross dipole antenna is formed of a first dipole antenna disposed at a prescribed interval on the reflecting plate, a second dipole antenna disposed at a prescribed interval on the reflecting plate so as to be approximately orthogonal to the first dipole antenna, and a plurality of non-feeding elements disposed around the first dipole antenna and second dipole antenna and uprising from the reflecting plate; the whip antenna is fixated on the reflecting plate by being isolated from the cross dipole antenna at more than approximately xc2xc wavelength of the wavelength in the center frequency of the used frequency band; and the whip antenna is also used as the non-feeding elements.
According to this type of invention, by disposing a plurality of non-feeding elements around the cross dipole antenna, it is possible to suppress the decrease of gain in a low elevation angle and to significantly improve the axial ratio characteristic of circularly polarized waves. In other words, the non-feeding elements act as the wave director and improve the antenna characteristic in the direction of the low elevation angle. Further, since a whip antenna, which is a ground antenna, can also be used as the non-feeding element, a composite antenna can be structured with only an approximate structure of a cross dipole antenna. The composite antenna can thereby be miniaturized.
Moreover, in the aforementioned composite antenna of the present invention, the first dipole antenna and second dipole antenna may be structured by being folded toward the reflecting plate.
Furthermore, in the aforementioned composite antenna of the present invention, the non-feeding elements may be fixated on the reflecting plate via insulation spacers.
Moreover, in the aforementioned composite antenna of the present invention, the reflecting face may be inclined such that the center portion of the reflection plate protrudes further than the peripheral portion. According to the above, it is possible to further suppress the decrease of gain in a low elevation angle and to significantly improve the axial ratio characteristic of circularly polarized waves.
Furthermore, in the aforementioned composite antenna of the present invention, the cross dipole antenna may be made to be capable of receiving broadcast signals of circularly polarized waves transmitted from a satellite and the whip antenna may be made to be capable of receiving broadcast signals of linearly polarized waves of identical contents as with the broadcast signals transmitted from the ground.
Moreover, in the aforementioned composite antenna of the present invention, the plurality of non-feeding elements are disposed circumferentially with the cross dipole antenna in the approximate center, and the whip antenna may be disposed on the outer side of the circumference.